Those functions provide means to execute commands on the system itself, and means to secure such commands.
No external libraries are needed to build this extension.
There is no installation needed to use these functions; they are part of the PHP core.
This extension has no configuration directives defined in php.ini.
This extension defines a process resource, returned by proc_open().
This extension has no constants defined.
Open files with lock (especially open sessions) should be closed before executing a program in the background.
These functions are also closely related to the backtick operator. Also, while in safe mode you must consider the safe_mode_exec_dir directive.